1. Field of the Invention
This invention relates generally to methods of producing single-wall carbon nanotubes, and to catalysts for use in such methods.
2. Description of Related Art
Fullerenes are closed-cage molecules composed entirely of sp2-hybridized carbons, arranged in hexagons and pentagons. Fullerenes (e.g., C60) were first identified as closed spheroidal cages produced by condensation from vaporized carbon.
Fullerene tubes are produced in carbon deposits on the cathode in carbon arc methods of producing spheroidal fullerenes from vaporized carbon. Ebbesen et al. (Ebbesen I), xe2x80x9cLarge-Scale Synthesis Of Carbon Nanotubes,xe2x80x9d Nature, Vol. 358, p. 220 (Jul. 16, 1992) and Ebbesen et al., (Ebbesen II), xe2x80x9cCarbon Nanotubes,xe2x80x9d Annual Review of Materials Science, Vol. 24, p. 235 (1994). Such tubes are referred to herein as carbon nanotubes. Many of the carbon nanotubes made by these processes were multi-wall nanotubes, i.e., the carbon nanotubes resembled concentric cylinders. Carbon nanotubes having multiple walls have been described in the prior art. Ebbesen II; Iijima et al., xe2x80x9cHelical Microtubules Of Graphitic Carbon,xe2x80x9d Nature, Vol. 354, p. 56 (Nov. 7, 1991).
Another known way to synthesize nanotubes is by catalytic decomposition of a carbon-containing gas by nanometer-scale metal particles supported on a substrate. The carbon feedstock molecules decompose on the particle surface, and the resulting carbon atoms then diffuse through the particle and precipitate as part of nanotubes growing from one side of the particle. This procedure typically produces imperfect multi-walled nanotubes in high yield. See C. E. Snyder et al., International Patent Application WO 89/07163 (1989), hereby incorporated by reference in its entirety. Its advantage is that it is relatively simple and can be scaled to produce nanotubes by the kilogram.
Single-wall carbon nanotubes have been made in a DC arc discharge apparatus of the type used in fullerene production by simultaneously evaporating carbon and a small percentage of Group VIII transition metal from the anode of the arc discharge apparatus. See Iijima et al., xe2x80x9cSingle-Shell Carbon Nanotubes of 1 nm Diameter,xe2x80x9d Nature, Vol. 363, p. 603 (1993); Bethune et al., xe2x80x9cCobalt Catalyzed Growth of Carbon Nanotubes with Single Atomic Layer Walls,xe2x80x9d Nature, Vol. 63, p. 605 (1993); Ajayan et al., xe2x80x9cGrowth Morphologies During Cobalt Catalyzed Single-Shell Carbon Nanotube Synthesis,xe2x80x9d Chem. Phys. Lett., Vol. 215, p. 509 (1993); Zhou et al., xe2x80x9cSingle-Walled Carbon Nanotubes Growing Radially From YC2 Particles,xe2x80x9d Appl. Phys. Lett., Vol. 65, p. 1593 (1994); Seraphin et al., xe2x80x9cSingle-Walled Tubes and Encapsulation of Nanocrystals Into Carbon Clusters,xe2x80x9d Electrochem. Soc., Vol. 142, p. 290 (1995); Saito et al., xe2x80x9cCarbon Nanocapsules Encaging Metals and Carbides,xe2x80x9d J. Phys. Chem. Solids, Vol. 54, p. 1849 (1993); Saito et al., xe2x80x9cExtrusion of Single-Wall Carbon Nanotubes Via Formation of Small Particles Condensed Near an Evaporation Source,xe2x80x9d Chem. Phys. Lett., Vol. 236, p. 419 (1995). It is also known that the use of mixtures of such transition metals can significantly enhance the yield of single-wall carbon nanotubes in the arc discharge apparatus. See Lambert et al., xe2x80x9cImproving Conditions Toward Isolating Single-Shell Carbon Nanotubes,xe2x80x9d Chem. Phys. Lett., Vol. 226, p. 364 (1994). While this arc discharge process can produce single-wall nanotubes, the yield of nanotubes is low and the tubes exhibit significant variations in structure and size between individual tubes in the mixture. Individual carbon nanotubes are difficult to separate from the other reaction products and purify.
High quality single-wall carbon nanotubes have also been generated by arc evaporation of a graphite rod doped with Y and Ni. See C. Journet et al., Nature 388 (1997) 756, hereby incorporated by reference in its entirety. These techniques allow production of only gram quantities of single-wall carbon nanotubes.
An improved method of producing single-wall nanotubes is described in U.S. Ser. No. 08/687,665, entitled xe2x80x9cRopes of Single-Walled Carbon Nanotubesxe2x80x9d incorporated herein by reference in its entirety. This method uses, inter alia, laser vaporization of a graphite substrate doped with transition metal atoms, preferably nickel, cobalt, or a mixture thereof, to produce single-wall carbon nanotubes in yields of at least 50% of the condensed carbon. See A. Thess et al. (1996), Science 273:483. The single-wall nanotubes produced by this method tend to be formed in clusters, termed xe2x80x9cropes,xe2x80x9d of 10 to 1000 single-wall carbon nanotubes in parallel alignment, held together by van der Waals forces in a closely packed triangular lattice. Nanotubes produced by this method vary in structure, although one structure tends to predominate. These high quality samples have for the first time enabled experimental confirmation of the structurally dependent properties predicted for carbon nanotubes. See J. W. G. Wildoer, L C. Venema, A. G. Rinzler, R. E. Smalley, C Dekker (1998), Nature, 391:59; T. W. Odom, J. L. Huang, P. Kim, C. M. Lieber (1998), Nature, 391:62.
Although the laser vaporization process produces improved single-wall nanotube preparations, the product is still heterogeneous, and the nanotubes are too tangled for many potential uses of these materials. In addition, the vaporization of carbon is a high-energy process and is inherently costly. Therefore, there remains a need for improved methods of producing single-wall nanotubes of greater purity and homogeneity. Furthermore, applications could make use of the properties of single-wall carbon nanotubes if only they were available in a form where they were attached directly to the surface of a macroscopic object. However, such components have not been produced up to now.
A method of producing carbon fibers from single-wall carbon nanotubes is described in PCT Patent Application No. PCT/US98/04513, incorporated herein by reference in its entirety. The single-wall nanotube molecules are produced in substantially two-dimensional array made up of single-walled nanotubes aggregating (e.g., by van der Waals forces) in substantially parallel orientation to form a monolayer extending in directions substantially perpendicular to the orientation of the individual nanotubes. Such monolayer arrays can be formed by conventional techniques employing xe2x80x9cself-assembled monolayersxe2x80x9d (SAM) or Langmiur-Blodgett films, see Hirch, pp. 75-76.
Typically, SAMs are created on a substrate which can be a metal (such as gold, mercury or ITO (indium-tin-oxide)). The molecules of interest, here the single-wall nanotube molecules, are linked (usually covalently) to the substrate through a linker moiety such as xe2x80x94Sxe2x80x94, xe2x80x94Sxe2x80x94(CH2)nxe2x80x94NHxe2x80x94, xe2x80x94SiO3(CH2)3NHxe2x80x94 or the like. The linker moiety may be bound first to the substrate layer or first to the single-wall nanotube molecule (at an open or closed end) to provide for reactive self-assembly. Langmiur-Blodgett films are formed at the interface between two phases, e.g., a hydrocarbon (e.g., benzene or toluene) and water. Orientation in the film is achieved by employing molecules or linkers that have hydrophilic and lipophilic moieties at opposite ends.
The production of single-wall carbon nanotubes by metal-catalyzed disproportionation of carbon monoxide has been reported. See Dai, et al. (1996), Chem. Phys. Lett., 260:471-475. Preformed catalyst particles were made from a 50:50 mixture of Ni/Co supported on fumed alumina nanoparticles using known methods (See Int. Pat. WO 89/07163 (1989)). The diameter of the single- or multi-wall nanotube structure growing from a catalyst particle is related to the dimensions of the catalyst particle itself. Using the known methods of catalyst particle preparation, it is not possible to provide nanoparticles with a uniform optimum size to produce only single-wall nanotubes, and the growth process of Dai, et al., does not provide high yields of single-wall nanotubes because the larger particles produce multiwall nanotubes.
This invention provides a method for predominant production of single-wall carbon nanotubes comprising: providing a supported transition metal catalyst supported on an inert surface contacted with a suitable feedstock gas (e.g. CO, or any of the known effective hydrocarbon gasses) at a temperature and pressure at which single-wall carbon nanotube growth occurs. Enhanced rates of production for single-wall nanotubes are provided by first placing catalyst material on an appropriate supporting substrate and treating the catalyst material so that it produces predominantly single-wall carbon nanotubes. At least initially, the conditions ensure that the reaction to form nanotubes is limited by the supply of carbon to the catalytic site, rather than by the rate of diffusion of carbon through the catalytic particle. This may be achieved via a chemical process in which the carbon contained in a controlled amount of feedstock gas interacts with catalyst particles. Under the appropriate conditions carbon in the feedstock gas is formed into single-wall nanotubes on the catalyst particles of less than 2-nanometer diameter but is formed into graphitic layers that encapsulate the larger catalyst particles, deactivating them as catalysts. Catalyst particles of greater than about 2 nanometers in diameter are more likely to form multiwall nanotubes, and since they are rendered ineffective by the process, the only remaining active catalyst particles support growth of primarily single-wall nanotubes. In a preferred embodiment, the method of this invention provides for treatment of supported catalyst material to deactivate catalyst particles that do not support growth of the desired nanotube types, with subsequent change of the feedstock composition or density to accelerate growth of the desired form of single-wall nanotubes. The method of this invention is capable of producing material that is  greater than 50% SWNT, more typically  greater than 90% SWNT, or even  greater than 99% SWNT.
This invention also provides a catalyst/support system structured so that access of the feedstock gas to the catalyst is enhanced by that structure. Preferably, the catalyst is deposited so that there is clear distance between catalyst locations by dispersion of small catalyst particles on the substrate surface or other methods of deposition known to those skilled in the art.
The production of high quality single-wall carbon nanotubes, in some cases including double-wall carbon nanotubes, in yields much larger than previously achieved by catalytic decomposition of carbon-containing precursor gases is disclosed. The nanotubes formed are connected to and grow away from the catalyst particles affixed to the catalyst support surface. If the growth time is short, the tubes can be only a fraction of one micron long, but if the growth time is prolonged, single-wall carbon nanotubes in this invention can grow continuously to arbitrary lengths. The present invention demonstrates a means for nucleating and growing nanotubes only from the smallest of the supported catalyst particles, which produce single-wall carbon nanotubes, while deactivating the larger particles so that no multi-walled nanotubes are produced. This allows the growth exclusively of single-wall carbon nanotubes from catalyst systems previously thought to produce only larger diameter multi-walled nanotubes.
According to one embodiment of the present invention, a process for producing single wall carbon nanotubes is disclosed. The process comprises the steps of: (1) providing a supported nanoscale particulate transition metal catalyst ill reaction zone; (2) supplying a gaseous carbon-containing compound to the reaction zone under conditions, at least initially, so that the compound inactivates catalyst particles that have a diameter large enough to catalyze the production of multi-wall carbon nanotubes; and (3) contacting the catalyst particles that have a diameter small enough to catalyze the production of primarily single-wall carbon nanotube which remain active under the conditions with the gaseous carbon-containing compound. The gaseous carbon-containing compound may be a hydrocarbon. In this case, the gaseous hydrocarbon may be supplied to the reaction zone at a rate that is low enough to cause the inactivation of larger diameter catalyst particles while causing the growth of single wall nanotubes from the smaller diameter catalyst particles. Under such conditions, it is believed the larger diameter catalyst particles are inactivated by encapsulation by graphitic layers before any multi-wall carbon nanotubes can grow therefrom. The gaseous carbon-containing compound may also be CO. In this case, the CO is contacted with the supported catalyst at a temperature and pressure that inactivates large diameter catalyst particles but produces single-wall carbon nanotubes in high yield. In either case, the conditions in the reaction zone may be changed, after the inactivation of the larger diameter catalyst particles, to conditions that favor the production of single-wall carbon nanotubes.
The catalyst may include nanoscale transition metal atom clusters supported on a substantially planar support. The transition metal atom clusters may be substantially uniformly disposed on the planar surface in close proximities to one another so that individual single-wall carbon nanotubes, or that bundles or ropes of generally aligned single-wall carbon nanotubes, grow from the supported catalyst particles. Changing the temperature in the reaction zone may selectively change the yield of single-wall carbon nanotubes.